The present invention is directed to removal of particulate matter from aqueous suspensions. In particular the invention concerns filtration of aqueous suspensions by means of ceramic membranes in cross flow operation.
A filter according to U.S. Pat. No. 4,946,592 coated with a yttrified zirconia film having a pore size of 5 xcexcm can provide a flow of approximately 600 l/hm2 for up 10 hours. The identity of the liquid was not specified. Another filter with a zirconia film having a pore size of 0.2 xcexcm provided a flow of the non-specified liquid approximately 140 600 l/h/m2 for up to 5 hours.
U.S. Pat. No. 4,698,157 concerns a filter membrane for filtering liquid e.g. foodstuffs and a method for preparing said filter. The method comprises the steps of preparing a deflocculated slip of a metal oxide comprising a dispersing agent and thickening agent.
The slip is poured into the tube. The tube is dried and subsequently calcined. The resulting filter membrane comprises a macroporous tube with a pore of 10-20 xcexcm, an additional layer inside of tube with a pore size of 1-3 xcexcm, and finally a filter (membrane) layer, which is 10 to 20 xcexcm thick and has a pore size of 0.2 to 0.8 xcexcm. The only filter layer exemplified in the disclosure is an alumina membrane calcined at 1300xc2x0 C. The membrane layer is characterised by an average roughness, which over a distance of five times the average grain size is less than one fifth of the average grain size.
U.S. Pat. No. 4,562,021 mentions two methods for preparing a medium for microfiltration. The first method comprises the step of preparing a slip made from e.g. TiO2, nitric acid, polyvinyl alcohol and an optional wetting agent. The slip is mixed and homogenised and thereafter filled into a porous ceramic tube. It is dried at 20xc2x0 C. for 24 hours and baked stepwise, e.g. up to 1200xc2x0 C.
According to the second method titanium-ethoxide is dissolved in an alcohol together with a gelling agent. The slip is filled into the tube for one to several minutes. The tube is dried in a humid atmosphere, whereby the titanium ethoxide is hydrolysed into Ti(OH)4. The tube is then calcined at 100xc2x0 C. to 1000xc2x0 C., whereby the hydroxide is decomposed into the oxide. The resulting microfiltration medium comprises an outer porous ceramic tube and an inner membrane of TiO2 with pore size ranges from 58 to 1100 xc3x85 and provide a water flow up to 5 l/h/m2.
EP Patent No. 645,174 A1 discloses a method for filtration of beer using an asymmetric ceramic membrane. Without any back-flush the flow rate decreased from 150 to 3 l/h/m2 within 2 hours and from 150 to 70 l/h/m2 with back-flush.
GB Patent No. 2,176,715 also discloses a method for cross-flow filtration of beer but without using back flush.
Q. Gan et al (Trans 1 ChemE, Vol 75, Part A, January 1997 pp. 3-8) have investigated beer clarification by cross-flow microfiltration using tubular ceramic membranes composed of alumina with 0.2, 0.5 and 1.3 xcexcm nominal pore diameters. Operating this type of membrane with raw beer a maximum average membrane flux of 22 kg/m2/h was obtained provided that suitable back-flush frequency and strength with regard to the base flux level was used.
Cross-flow microfiltration may be an economical replacement for kieselguhr filters. Burell and Ried (Filtration and Separation June 1994, pp. 399-405) also found that 0.5 xcexcm ceramic cross flow microfiltration membranes impart a high degree of both clarity and micro-organism removal to the filtrate. They obtained an average membrane flux of up to 53 l/m2/h for 5 hour filtration periods falling to 35 l/m2/h after 15 hours.
JP 58 101 718 discloses filtration of a suspension with particles of two sizes, where the bigger particles are to be removed by a filter of synthetic polymer material. The smaller particles are positively charged and the filter is applied a material, which makes the surface positive and the smaller particles passing through the filter are repelled from the filter and clogging is prevented.
U.S. Pat. No. 4,888,115 discloses filtration, where the polarity of the Zeta potential of the particles in a suspension and the filter surface is made to be the same to prevent clogging of the filter. The filter is an organic polymeric membrane with a charge modifying agent bound to the membrane microstructure, chosen to obtain the required polarity.
The general object of this invention is to provide a method and membrane with high and stable average permeate flux of aqueous suspensions with inorganic and organic particles. Aqueous suspensions of the above type are typically in food and beverage processing including clarification of wine and beer. In filtration of wine and beer yeast cells, cell de-bries and larger proterinaceous compounds are a particular problem, and extensive deposition of proteins, carbohydrates and minerals on the filter material largely influence product qualities. In beer clarification processes a most usually employed filter aid is kieselguhr. The disadvantage of kieselguhr is frequent regeneration and disposal of used spent kieselguhr.
Use of ceramic filters in filtration of beer is known in the art and discussed herein before. The known ceramic filters consist typically of sintered metal and micro-oporus glass and are operated with cross-flow. Those filters are not compressible and may be exposed to aggressive chemical environments. However, the known ceramic filters are limited by low permanent flux and by essential quality component retention. Both phenomena arise from severe filter membrane fouling which involves progressive pore flooding, in depth is adsorption/deposition, concentration polarisation and filtration layer formation.
Irreversible fouling of the ceramic membrane is a major problem in filtration and clarification of aqueous suspensions with particulate matter having high affinity to the ceramic material employed in the filter membrane.
It has been found that when adjusting the Zeta partical of a filter membrane layer to the same sign of polarity of particles to be filtered, fouling of the membranes is completely reversible and the membranes show high permeate flux at long time operation.
Periodically back-flushing is an effective method to control reversible fouling and ensures a maximum available membrane surface and high flux rates.
Pursuant to the above findings and observations, this invention provides a method for the removal of particulate matter from aqueous suspension comprising steps of
establishing value of pH and of Zeta potential of particles in the suspension;
providing a porous ceramic filter having a membrane layer consisting of at least a metal-oxide with a Zeta potential at the pH value of the suspension having same polarity of the Zeta potential as the particles in the suspension passing the suspension through the porous filter and withdrawing a filtrate.
Zeta potential defines the electrical charge on particles surface in aqueous suspensions. The excess charge at the steam surface of a double layer surrounding the particles in aqueous suspension is measured with a Zeta potential meter by means of known electro-foresis, electro-osmosis and flow of sedimentation potential measurements.
Depending on the suspended particles surface properties, many aqueous suspensions exhibit correlations between system variable and Zeta potential. An important variable is pH value of the aqueous suspension. Thus, to operate the invention in proper manner, it will be necessary to monitor pH value of the suspension to be filtered and optionally to adjust the pH of the suspension to a value at which the desired Zeta potential is obtained. At a Zeta potential with the same signs as the surface of the filtering membrane depth ad adhesion of particles is substantially reduced through repulsion forces. Owing repulsion and weakened adhesion in the pore surface of the filtering membrane fouling of the membrane is substantially reversed through back-flush of the membrane with recovered permeate.
Typically the membranes will be operated in cross-flow manner. Collected particulate matter is thereby continuously removed in tangential flow to the membrane surface. The driving force in cross-flow filtration is either gravity or pressure of the suspension flowing through the membrane. Membrane employed in filtration of colloidal solutions and suspensions have typically a pore size of between 0.3 and 20 micrometers. The desired pore size and pore size distribution is obtained by proper selection of the appropriate starting material and preparation process in accordance with known principals in membrane preparation.
Procedures for producing membranes of fine oxide powder, being useful in the inventive method include calcination of oxidic fine powders, deposition of a suspension of the calcined powder on a substrate and finally calcination of the thus prepared supported membrane. Selection of starting materials for the preparation of the calcined powder material depends on the pH value and on the sign of polarity of the Zeta potential in the aqueous suspension to be subjected to filtration. The most suitable materials are oxides of Al, Ti, Zr, Si and W together with magnesium-aluminum spinel with different crystal structures. Those oxides have a Zeta potential at different pH values as summarised in Table 1 below:
To obtain uniform and pure particles, the selected oxidic powder is calcined. Different atom structures or crystal phases obtained in some of the oxidic powders are determined by the calcination temperature.
By calcining titania prior to casting of the membrane it is ensured that all the metal oxide is in the most stable form. Thereby very uniform and stable particles are obtained. Furthermore, the particles have a monodispers grain size distribution.
Calcination of the MeOn particles prior to slip casting in combination with repeated slip casting of the membrane provides a high uniformity of the pore size distribution in the final membrane filter.
Membrane tubes made of unsupported TiO2 have low tendency to filter clogging, in particular when filtering beer.
Filters made as ceramic filters applied with calcinated metal oxide membranes have a high mechanical and thermal stability and are especially useful in food and beverage industry as they can be sterilised at high temperatures. Furthermore, an additional degree of freedom in choice of membrane material is obtained due to the relationship of this invention between the polarity of the metal oxides and pH in the suspension.